1. Field of the Invention
The present invention relates to a magnetic recording medium, and a coating material having excellent storage stability for forming a backing layer of a magnetic recording medium. The backing layer has excellent durability, surface smoothness, light shielding, and electric conductivity.
2. Description of the Related Arts
In magnetic recording media, such as video tape of high grades, distal audio tape, and tape for computers, a backing layer is generally formed on a surface of a supporter opposite to a surface having a magnetic layer. The backing layer must provide running stability, prevent therefor generation of static electricity, and shield light so as to detect the end of the tape. A desired filler such as carbon black is usually dispersed in the backing layer to provide electric conductivity and shield light.
While recording density has been increasing in recent years and smoothness of a magnetic layer has been improved, improvement in smoothness of its backing layer is increasingly necessary. When durability and smoothness of the backing layer are insufficient, uneveness of the backing layer is transferred to the magnetic layer, and thus problems such as drop-out and decrease in its SN ratio arise.
While magnetic tape has been made thinner in recent years, improvement in electric conductivity and light shielding of the backing layer are also increasingly necessary. In order to obtain therefor sufficient durability, smoothness of surface, electric conductivity, and light shielding of the backing layer, a filler such as carbon black having a relatively small particle size must be dispersed at a higher concentration with a higher degree of dispersion. However, carbon black having a smaller particle size generally has a larger specific surface area, higher oil absorption, and is difficult to disperse. Furthermore, a coating material prepared by dispersing such carbon black shows poor coating properties due to increased viscosity after long-term storage because of inferior storage stability. Re-aggregation of the filler, such as carbon black, in the coating material also takes place and has an adverse effect on smoothness of the surface of the backing layer.
Improvement in dispersion and stability of a coating material has been attempted by using a resin having a polar group, such as a hydroxyl group, a carboxyl group, or the like. However, sufficient smoothness of a coating layer and sufficient stability of a coating material have not been achieved yet.
In Japanese Patent Application Laid-Open (JP-A) No. 60(1985)-229227, use of a copper salt of a fatty acid having 12 or more carbon atoms was attempted for improving the running property of a coating layer. However, it was found that Young's modulus of the coating layer decreased under the conditions of high temperature and high humidity, and durability of the coating layer deteriorated.
The present invention provides a solution for the problems described above.
Accordingly, an object of the present invention is to provide a magnetic recording medium comprising a backing layer having excellent durability and smoothness of the surface thereof.
Another object of the present invention is to provide a magnetic recording medium which comprises a backing layer having excellent electric conductivity and light shielding and which has an excellent magnetic property as a whole.
Still another object of the present invention is to provide a coating material for forming a backing layer which is suitable for producing a magnetic recording medium such as described above and has improved dispersion of a filler and storage stability.